This invention relates to a switch used in electronic systems in which the voltage across a capacitor is amplified by a trans-capacitance operational amplifier.
Electronic systems which process the voltage provided by a sensing device approximately modeled by a capacitor (such as a MEMS gyroscope or a MEMS microphone) use a voltage amplifier implemented as a continuous' time trans-capacitance operational amplifier. The operational amplifier feedback path comprises a capacitor in series with a switch. It is desirable that the switch is connected to the input of the operational amplifier (and hence to the sensing device), while the capacitor is connected to the output of the operational amplifier. The sensing device provides a very low voltage, so it is critical that the current leakage due to the imperfections of the switch in the “off” state is minimized. Therefore, the switch used in the sampling circuitry requires a very high impedance (>1 TOhm) in the “off” state, and a low impedance (<1 kOhm) in the “on” state.
Conventional techniques use one or two MOS transistors to implement the switch. These transistors must have a ratio W/L high enough to allow a low impedance in the “on” state. However, the larger this ratio is, the larger the leakage of the switch becomes in the “off” state. Such conventional techniques can only achieve an equivalent “off” impedance on the order of a few GOhms. To avoid the unacceptable leakage from the sensing device, conventional switches are connected at the output of the operational amplifier. This solution has significant disadvantages, such as the increase of the noise of the amplifier due to the switching of the capacitor in parallel to the sensing device.